very simple signal generators based on 555 timer chips, generating square waves or "rectangle waves" with adjustable frequency or duty cycle or both.

slightly more complex "analog" triangle, sine, square generators, often with an analog input that is used to modulate the output duty cycle (PWM), frequency (FM), amplitude (AM), or phase (PM). A special case is DTMF generators. Alas, many of the ICs used historically to build such "analog" signal generators and DTMF generators are no longer being manufactured. "Analog function generator ICs": XR2206, XR2209, ICL8038 are apparently now obsolete.

See Also

The Open Bench project is developing a set of open source benchtop tools for the electronics workbench -- adjustable DC power supply; LCR meter; waveform generator; frequency counter; logic analyzer; oscilloscope.